Glass for data storage medium substrate, glass substrate for data storage medium and magnetic disk

ABSTRACT

To provide glass for a data storage medium substrate, whereby high heat resistance can be obtained. 
     Glass for a data storage medium substrate, which comprises, as represented by mol percentage based on the following oxides, from 55 to 70% of SiO 2 , from 2.5 to 9% of Al 2 O 3 , from 0 to 10% of MgO, from 0 to 7% of CaO, from 0.5 to 10% of SrO, from 0 to 12.5% of BaO, from 0 to 2.5% of TiO 2 , from 0.5 to 3.7% of ZrO 2 , from 0 to 2.5% of Li 2 O, from 0 to 8% of Na 2 O, from 2 to 8% of K 2 O and from 0.5 to 5% of La 2 O 3 , provided that the total content of Al 2 O 3 and ZrO 2 (Al 2 O 3 +ZrO 2 ) is at most 12%, and the total content of Li 2 O, Na 2 O and K 2 O (R 2 O) is at most 12%.

TECHNICAL FIELD

The present invention relates to a glass substrate to be used for a datastorage medium such as a magnetic disk or an optical disk, glasstherefor and a magnetic disk.

BACKGROUND ART

As glass for a data storage medium substrate such as a magnetic disk oran optical disk, for example, lithium-containing aluminosilicate glasshaving a high Young's modulus or one having chemical reinforcingtreatment applied thereto (e.g. Patent Document 1), or crystallizedglass having a crystal layer precipitated by heat-treating glass havinga specific composition (e.g. Patent Document 2), is used.

In recent years, along with an increase of the storage capacity of ahard disk drive, development for high recording density has been inprogress at a high pace. However, along with the development for highrecording density, microsizing of magnetic particles tends to impair thethermal stability, and crosstalk or a decrease in the SN ratio ofplayback signals is problematic. Here, a heat-assisted magneticrecording technology has attracted attention as a united technique oflight and magnetism. This is a technique wherein a magnetic recordinglayer is irradiated with a laser beam or near-field light to have thecoercive force locally reduced at the heated portion, and in such astate, an external magnetic field is applied for recording, and therecorded magnetization is read out by e.g. a GMR element. Thus,recording can be made on a high-coercive force medium, whereby itbecomes possible to microsize the magnetic particles while maintainingthe thermal stability. However, in order to form such a high-coerciveforce medium in the form of a multilayer film, the substrate is requiredto be sufficiently heated, and a highly heat resistant substrate isrequired.

Patent Document 1: JP-A-2001-180969

Patent Document 2: JP-A-2000-119042

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Substrate glass for chemical reinforcing treatment has heat resistanceadjusted to be low so that chemical reinforcing treatment canefficiently be applied, and is likely to be buckled by heating at thetime of forming the above-mentioned high-coercive force medium in theform of a multilayer film. Further, when a chemically reinforced glasssubstrate is subjected to the above-mentioned heating, the ionexchange-treated layer is likely to be diffused by such heating, wherebythe strength is likely to be deteriorated.

Further, if it is attempted to employ a crystallized glass substrate,the substrate surface is likely to be distorted by the above-mentionedheating due to a difference in the thermal expansion coefficient betweenthe crystalline layer and the bulk body.

It is an object of the present invention to provide glass for a datastorage medium substrate, whereby high heat resistance can be obtainedeven if no chemical reinforcing treatment or crystallizing treatment isapplied, a glass substrate for a data storage medium, and a magneticdisk.

Means to Solve the Problems

The present invention provides glass for a data storage medium substrate(hereinafter referred to as the glass of the present invention), whichcomprises, as represented by mol percentage based on the followingoxides, from 55 to 70% of SiO₂, from 2.5 to 9% of Al₂O₃, from 0 to 10%of MgO, from 0 to 7% of CaO, from 0.5 to 10% of SrO, from 0 to 12.5% ofBaO, from 0 to 2.5% of TiO₂, from 0.5 to 3.7% of ZrO₂, from 0 to 2.5% ofLi₂O, from 0 to 8% of Na₂O, from 2 to 8% of K₂O and from 0.5 to 5% ofLa₂O₃, provided that the total content of Al₂O₃ and ZrO₂ (Al₂O₃+ZrO₂) isat most 12%, and the total content of Li₂O, Na₂O and K₂O (R₂O) is atmost 12%.

Further, the present invention provides glass for a data storage mediumsubstrate (hereinafter referred to as the first glass or the glass 1),which comprises, as represented by mol percentage based on the followingoxides, from 55 to 70% of SiO₂, from 2.5 to 9% of Al₂O₃, from 0 to 10%of MgO, from 0 to 7% of CaO, from 1 to 10% of SrO, from 0.5 to 12.5% ofBaO, from 0 to 2.5% of TiO₂, from 0.5 to 3.7% of ZrO₂, from 0 to 2.5% ofLi₂O, from 0 to 5% of Na₂O, from 2 to 8% of K₂O and from 0.5 to 5% ofLa₂O₃, provided that Al₂O₃+ZrO₂ is at most 12%, the difference obtainedby subtracting the Li₂O content from the total content of SrO and BaO isat least 2%, and R₂O is at most 10%.

Further, the present invention provides glass for a data storage mediumsubstrate (hereinafter referred to as the second glass or the glass 2),which comprises, as represented by mol percentage based on the followingoxides, from 60 to 70% of SiO₂, from 2.5 to 9% of Al₂O₃, from 2 to 10%of MgO, from 0 to 7% of CaO, from 0.5 to 10% of SrO, from 0 to 5% ofBaO, from 0 to 1% of TiO₂, from 1 to 3.7% of ZrO₂, from 0 to 2.5% ofLi₂O, from 2 to 8% of Na₂O, from 2 to 8% of K₂O and from 0.5 to 3% ofLa₂O₃, provided that Al₂O₃+ZrO₂ is at most 12%, the total content ofMgO, CaO, SrO and BaO (RO) is at least 10 mol %, and R₂O is at most 12%.

Further, the present invention provides a glass substrate for a datastorage medium, which is made of the above glass for a data storagemedium substrate.

Further, the present invention provides a magnetic disk having amagnetic recording layer formed on the above glass substrate for a datastorage medium.

Effects of the Invention

According to the present invention, it becomes possible to obtain ahighly heat resistant glass substrate for a data storage medium suitablefor a heat-assisted magnetic recording even without applying chemicalreinforcing treatment or crystallization treatment.

According to a preferred embodiment of the present invention, it ispossible to obtain glass having a small content of an alkali metaloxide, whereby an improvement of the weather resistance of a glasssubstrate can be expected.

Further, it becomes possible to obtain a glass substrate having athermal expansion coefficient comparable with a conventional glasssubstrate.

Further, it becomes possible to use a float process, which is useful formass production of a glass substrate.

Further, it becomes possible to obtain glass having a Young's modules(E) of at least 75 GPa, whereby the glass substrate tends to be scarcelydeflected, and it becomes possible to increase the storage capacity.Even when the storage medium is subjected to a shock, it tends to behardly breakable.

Further, when E is adjusted to be at most 90 GPa, the polishingprocessing of the glass substrate tends to be easy.

Further, it becomes possible to minimize the surface roughness of glassafter the final polishing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relation between a value obtained bydividing a product of the Al₂O₃ content multiplied by the total contentof ZrO₂, K₂O and La₂O₃, by the total content of MgO, CaO, SrO and BaO(Al(ZrKLa)/RO), and the difference ΔT obtained by subtracting the liquidphase temperature from the temperature where the viscosity becomes 10⁴dPa·s.

FIG. 2 is a graph showing the relation between a value obtained bydividing a product of the sum of the MgO and CaO contents multiplied bythe sum of the La₂O₃ and TiO₂ contents, by the ZrO₂ content((((MgO+CaO)×(La₂O₃+TiO₂))÷ZrO₂), unit: mol %), and the scattered peakintensity (optional unit) obtained by carrying out the measurement ofsmall-angle X-ray scattering of the glass.

FIG. 3 is a graph showing the relation between the scattered peakintensity (optional unit) obtained by carrying out the measurement ofsmall-angle X-ray scattering of the glass, and the surface roughness Ra(unit: nm) of the polished glass, as measured by an atomic forcemicroscope.

BEST MODE FOR CARRYING OUT THE INVENTION

The glass substrate for a data storage medium in the present inventionis typically a circular glass plate having a thickness of from 0.5 to1.5 mm and a diameter of from 48 to 93 mm, and in the case of a magneticdisk glass substrate or the like, a hole having a diameter of from 15 to25 mm is usually formed at its center.

In a magnetic disk of the present invention, at least a magnetic layerbeing a magnetic recording layer is formed on the main surface of theglass substrate for a data storage medium of the present invention, andin addition, an underlayer, a protective layer, a lubricating layer or aroughness-controlling layer may, for example, be formed as the caserequires.

A process for producing the glass substrate for a data storage medium ofthe present invention is not particularly limited. For example,materials for the respective components commonly used may be blended tohave a desired composition, such a mixture is heated and melted in aglass melting furnace, the molten glass is homogenized by e.g. bubbling,stirring or addition of a clarifier, followed by forming into a plateglass having a predetermined thickness made of the glass for a datastorage medium substrate of the present invention, by a well knownmethod such as a float process, a press method or a Down-draw method.Then, after annealing, processing such as grinding or polishing may becarried out to obtain a glass substrate having prescribed size andshape. As the forming method, a float process suitable for massproduction is preferred.

The glass transition temperature (Tg) of the glass of the presentinvention is preferably at least 660° C. If it is lower than 660° C., ittends to be difficult to apply the above-mentioned heat-assistedmagnetic recording technique to be carried out at a high heat treatingtemperature to form a magnetic layer. It is more preferably at least680° C., particularly preferably at least 690° C., and especially whenit is desired to increase the heat resistance, it is preferably at least700° C. Tg is typically at most −740° C.

The glass of the present invention preferably has an average linearexpansion coefficient (α) of at least 75×10⁻⁷/° C. within a range offrom 50 to 350° C. If α is less than 75×10⁻⁷/° C., it is smaller than αof conventional glass, and on the other hand, α of the metal of the hubto be attached to the substrate is typically at least 100×10⁻⁷/° C.,whereby the difference in α between the hub and the glass substratebecomes large, and the glass substrate is likely to be easily broken. αis more preferably at least 76×10⁻⁷/° C., particularly preferably atleast 77×10⁻⁷/° C., most preferably at least 78×10⁻⁷/° C., and typicallyat most 90×10⁻⁷/° C.

With the glass of the present invention, the difference between thetemperature (T₄) at which the viscosity becomes 10₄ dPa·s and the liquidphase temperature (T_(L)) i.e. ΔT(=T₄−T_(L)) is preferably at least −70°C. If the difference is less than −70° C., forming into a glass platetends to be difficult.

ΔT is more preferably at least 0° C. If ΔT is less than 0° C., formingby a float process tends to be difficult. ΔT is particularly preferablyat least 10° C., most preferably at least 20° C.

The density (d) of the glass of the present invention is preferably atmost 3.3 g/cm³. If d exceeds 3.3 g/cm³, weight reduction of the datastorage medium tends to be difficult, the power consumption required fordriving the recording medium tends to increase, the disk tends to bevibrated by an influence of a windage loss during its rotation thusleading to an error in reading out, or when a recording medium receivesa shock, the substrate tends to be easily deflected and is likely to bebroken by formation of a stress. Thus, d is more preferably at most 3.0g/cm³, most preferably at most 2.8 g/cm³. Of the glass 2, d is typicallyat most 2.75 g/cm³.

The Young's modulus (E) of the glass of the present invention ispreferably from 75 to 90 GPa. If E is less than 75 GPa, the disk tendsto be vibrated by an influence of a windage loss during its rotationthus leading to an error in reading out, or when a recording mediumreceives a shock, the substrate tends to be easily deflected and islikely to be broken by formation of a stress. E is more preferably atleast 78 GPa, most preferably at least 80 GPa. If E exceeds 90 GPa, thepolishing rate tends to be low, or a local stress is likely to be formedwhereby breakage is likely to result, and it is typically at most 87GPa.

E/d is typically from 25 to 35 MN/kg, and in the case of the glass 2, itis typically from 28 to 33 MN/kg.

The above-mentioned first glass is glass of the present invention and isone wherein SrO is at least 1 mol %, BaO is at least 0.5 mol %, Na₂O isfrom 0 to 5 mol %, the difference obtained by subtracting the Li₂Ocontent from the total content of SrO and BaO is at least 2 mol %, andR₂O is at most 10 mol %.

This first glass is suitable in a case where e.g. T₄ is desired to belowered, ΔT is desired to be increased, or Tg is desired to be madehigh.

The above-mentioned second glass is glass of the present invention andis one wherein SiO₂ is at least 60 mol %, MgO is at least 2 mol %, BaOis from 0 to 5 mol %, TiO₂ is from 0 to 1 mol %, ZrO₂ is from 1 to 3.7mol %, Na₂O is at least 2 mol %, La₂O₃ is at most 3%, and RO is at least10 mol %.

This second glass is suitable in a case where e.g. d or E/d is desiredto be made small, or phase separation is desired to be made to hardlyoccur.

Now, the composition of the glass of the present invention will bedescribed. Here, the contents of the respective components arerepresented by mol percentage unless otherwise specified.

SiO₂ is an essential component to form the skeleton of the glass. IfSiO₂ is less than 55%, the glass tends to be unstable, or the weatherresistance or acid resistance tends to deteriorate. SiO₂ is preferablyat least 57%, typically at least 60%. If SiO₂ exceeds 70%, α tends to betoo small, or the melting temperature to prepare the glass tends to betoo high. SiO₂ is preferably at most 68%, typically at most 67%.

In the glass 2, SiO₂ is at least 60%. If SiO₂ is less than 60%, theglass tends to be unstable, the glass tends to undergo phase separation,or Tg, weather resistance or acid resistance tends to deteriorate. SiO₂is preferably at least 61%, typically at least 63%.

In the glass 2, the content of SiO₂ as represented by mass percentage istypically less than 60.2%.

Al₂O₃ has an effect to increase Tg, weather resistance or E and is anessential component. If Al₂O₃ is less than 2.5%, the above effect tendsto be small, and it is preferably at least 4%. If Al₂O₃ exceeds 9%, ZrO₂type or K—Al—Si type crystals are likely to precipitate, and T_(L) islikely to be high, and it is preferably at most 8%. Further, in theglass 1, the Al₂O₃ content represented by mass percentage is typicallyless than 11%, more typically at most 10.5%.

MgO is not an essential component, but has an effect to lower theviscosity of the molten glass thereby to facilitate melting of theglass, to reduce d or to increase E, and thus it may be contained in anamount of up to 10%. If MgO exceeds 10%, the chemical durability tendsto deteriorate, the glass tends to be unstable, or T_(L) tends to be toohigh, and further, the glass is likely to undergo phase separation. Andaccordingly, it is preferably at most 9%. In a case where MgO iscontained, it is preferably contained in an amount of at least 1%. Inthe glass 2, MgO is particularly preferably at most 9%.

In the glass 2, MgO is an essential component. If MgO is less than 2%,the viscosity of the molten glass tends to be so high that the glasstends to be hardly melted, d tends to be large, or E tends to be low.MgO is preferably at least 4%.

CaO is not an essential component, but has an effect to lower theviscosity of the molten glass to facilitate melting of the glass, or toincrease E, and it may be contained up to 7%. If CaO exceeds 7%, ittends to form crystals with Si or La, T_(L) is likely to be high, theglass tends to be unstable, or the chemical durability tends todeteriorate, and further, the glass is likely to undergo phaseseparation. CaO is preferably at most 6%, and in the glass 2, it istypically at most 4%. In a case where CaO is contained, the content istypically at least 1%.

SrO has an effect to increase α, to lower the viscosity of the moltenglass thereby to facilitate melting of the glass, or to lower T_(L) andis essential. If SrO is less than 0.5%, the above effect tends to besmall, and it is preferably at least 1%, more preferably at least 2%. IfSrO exceeds 10%, the chemical durability tends to be low, the glasstends to be unstable, or d tends to be too large, and it is preferablyat most 8%.

In the glass 1, if SrO is less than 1%, the above effect tends to below, and SrO is at least 1%, preferably at least 2%.

BaO is not an essential component, but has an effect to increase α, tolower the viscosity of the molten glass thereby to facilitate melting ofthe glass, or to lower T_(L), and it may be contained in an amount of upto 12.5%. If BaO exceeds 12.5%, Ba—Al—Si type crystals are likely to beformed, whereby T_(L) is rather likely to be high, the weatherresistance tends to deteriorate, d tends to be too large, or E tends todeteriorate, and it is preferably at most 10%. In a case where BaO iscontained, the content is typically at least 0.1%.

In the glass 1, BaO is essential. If BaO is less than 0.5%, theviscosity of the molten glass tends to be high, whereby the glass tendsto be hardly melted, or T_(L) tends to be high, and it is preferably atleast 1%.

In the glass 2, if BaO is contained, the content is at most 5%. If BaOexceeds 5%, d is likely to be too large, or E tends to deteriorate, andit is preferably at most 3%.

The total content RO of MgO, CaO, SrO and BaO is preferably at least10%. If RO is less than 10%, α is likely to be too small, and it istypically at least 12%.

On the other hand, RO is preferably at most 30%. If RO exceeds 30%, Tgis likely to be low, or d is likely to be large, and it is preferably atmost 25%, typically at most 22%.

In the glass 2, RO is required to be at least 10%. If RO is less than10%, α is likely to be too small, and it is typically at least 12%.

TiO₂ is not an essential component, but has an effect to increase Tg,weather resistance or E, and thus, it may be contained in an amount ofup to 2.5%. If TiO₂ exceeds 2.5%, the glass is likely to be unstable,and it is preferably at most 2%. In a case where TiO₂ is contained, thecontent is typically at least 0.5%.

In the glass 1, the total content of MgO, CaO and TiO₂ is preferably atmost 16%. If the total content exceeds 16%, phase separation is likelyto take place in a case where the annealing temperature is high at thetime of annealing the glass or in a case where the cooing rate is slow.

In the glass 2, in a case where TiO₂ is contained, the content is atmost 1%. If TiO₂ exceeds 1%, the glass is likely to undergo phaseseparation, and it is preferably at most 0.9%, more preferably at most0.5%. In order to suppress phase separation, it is preferred not tocontain TiO₂.

ZrO₂ has an effect to increase Tg, the weather resistance or E and thusis essential. If it is less than 0.5%, the above effect tends to besmall, and it is preferably at least 1%. If ZrO₂ exceeds 3.7%, ZrO₂crystals are likely to precipitate, T_(L) is likely to be high, or d islikely to be too large, and it is preferably at most 3.6%.

If the total content of Al₂O₃ and ZrO₂ (Al₂O₃+ZrO₂) exceeds 12%, T_(L)is likely to be too high, and the total content is typically at most11%.

In the glass 2, ZrO₂ is at least 1%. If ZrO₂ is less than 1%, theweather resistance is likely to deteriorate, E tends to be small, orphase separation is likely to take place.

In the glass 2, the ratio of the content of SiO₂ represented by masspercentage to the sum of the TiO₂ and ZrO₂ contents represented by masspercentage (SiO₂/(TiO₂+ZrO₂)) is preferably more than 10. If the ratiois 10 or less, T_(L) is likely to be high, and the ratio is preferablyat least 12.

Li₂O is not an essential component, but has an effect to increase α, tolower the viscosity of the molten glass thereby to facilitate melting ofthe glass, or to increase E, and may be contained in an amount of up to2.5%. If Li₂O exceeds 2.5%, Tg is likely to substantially decrease, orthe weather resistance is likely to deteriorate, and in the glass 2,phase separation is likely to take place in addition, and it ispreferably at most 1%, and typically preferably no Li₂O is contained.

In the glass 1, the difference obtained by deducting the Li₂O contentfrom the total content of SrO and BaO [(SrO+BaO)−Li₂O] is at least 2%.If this difference is less than 2%, Tg is likely to be too low, and thedifference is preferably at least 3.5%.

Na₂O is not an essential component, but has an effect to increase α, orto lower the viscosity of the molten glass thereby to facilitate meltingof the glass, and thus may be contained in an amount of up to 8%. IfNa₂O exceeds 8%, Tg of the weather resistance is likely to deteriorate,and it is preferably at most 6%.

In the glass 1, when Na₂O is contained, the content is at most 5%. IfNa₂O exceeds 5%, Tg or the weather resistance is likely to deteriorate,and it is preferably at most 4%. In the glass 2, Na₂O is an essentialcomponent. If Na₂O is less than 2%, α tends to be small, or theviscosity of the molten glass is likely to be large whereby the glasstends to be hardly melted, and it is preferably at least 3%.

K₂O has an effect to increase α or to lower the viscosity of the moltenglass thereby to facilitate melting of the glass and is essential. IfK₂O is less than 2%, such an effect tends to be small, and it ispreferably at least 3%. If K₂O exceeds 8%, K—Si—Al type crystals arelikely to be formed, whereby T_(L) is likely to be high, or the weatherresistance or E is likely to be low, and it is preferably at most 7%.

If the total content of Li₂O, Na₂O and K₂O i.e. R₂O exceeds 12%, Tg orthe weather resistance is likely to deteriorate, and it is preferably atmost 10%.

R₂O is preferably at least 5%. If it is less than 5%, α tends to be toosmall, or the viscosity of the glass is likely to be high.

In the glass 1, R₂O is at most 10%. If R₂O exceeds 10%, Tg or theweather resistance is likely to deteriorate, and it is preferably atmost 9%.

The molar ratio of the total content of Li₂O and Na₂O to the K₂O contenti.e. (Li₂O+Na₂O)/K₂O is preferably at most 3. If the molar ratio exceeds3.0, Tg is likely to be too low, or the effect to lower Tg tends to benot negligible rather than the effect to increase α, and it ispreferably at most 1.4, typically at most 1.1. In the glass 1, such amolar ratio is preferably at most 1.4.

La₂O₃ has an effect to increase Tg, to increase α or to increase E andthus is essential. If La₂O₃ is less than 0.5%, such an effect tends tobe small, and it is preferably at least 1%. If La₂O₃ exceeds 5%, Si—Latype crystals or Si—Ca—La type crystals are likely to be formed, wherebyT_(L) is likely to be high, or d is likely to be too large, and it ispreferably at most 3%.

In the glass 2, La₂O₃ is at most 3%. If La₂O₃ exceeds 3%, Si—La typecrystals or Si—Ca—La type crystal are likely to be formed, whereby T_(L)is rather likely to be high, d is likely to be large, or phaseseparation is likely to take place, and it is preferably at most 2%.

In connection with the glass 1, FIG. 1 is a graph wherein a valueobtained by dividing a product of the Al₂O₃ content multiplied by thetotal content of ZrO₂, K₂O and La₂O₃, by the total content of MgO, CaO,SrO and BaO i.e. Al(ZrKLa)/RO (abscissa, unit: mol %), and ΔT=T₄−T_(L)(ordinate, unit: ° C.) were plotted with respect to 67 types of glasswith compositions comprising, as represented by mol percentage, from 52to 67.5% of SiO₂, from 4 to 15% of Al₂O₃, from 0 to 8.5% of MgO, from 0to 8% of CaO, from 0.5 to 9% of SrO, from 0 to 10% of BaO, from 0 to 4%of TiO₂, from 0 to 4% of ZrO₂, from 0 to 2.5% of Li₂O, from 0 to 6% ofNa₂O, from 3 to 7% of K₂O and from 0 to 5% of La₂O₃. Here, with respectto T₄ of some of glass, values calculated from their compositions wereemployed.

From FIG. 1, it is evident that Al(ZrKLa)/RO should preferably be madeto be at most 4.0% when it is desired to adjust ΔT to be at least 0° C.If Al(ZrKLa)/RO exceeds 4.0%, ΔT is likely to be less than 0° C.,whereby forming by a float process is likely to be difficult. It is morepreferably at most 3.5%, particularly preferably at most 3.0%.

In the glass 1, in a case where the total content of Al₂O₃ and ZrO₂ isat most 9%, the total content of Al₂O₃, ZrO₂, K₂O and La₂O₃ ispreferably at most RO. Otherwise, Si—Al—La type crystals, ZrO₂ typecrystals or Si—K—La type crystals are likely to precipitate, whereby ΔTis likely to be small.

In the glass 1, in a case where the total content of Al₂O₃ and ZrO₂exceeds 9%, the total content of ZrO₂ and K₂O is preferably less than9%. Otherwise, ZrO₂ type crystals or Si—K—La type crystals are likely toprecipitate, whereby ΔT is likely to be small.

In connection with the glass 2, FIG. 2 is a graph wherein a valueobtained by dividing a product of the sum of the MgO and CaO contentsmultiplied by the sum of the La₂O₃ and TiO₂ contents, by the ZrO₂content i.e. (Mg+Ca)(La+Ti)/Zr (abscissa, unit: mol %), and thescattered peak intensity obtained by carrying out the measurement ofsmall-angle X-ray scattering of the glass i.e. SAXS intensity (ordinate,optional unit) were plotted with respect to 20 types of glass withcompositions comprising, as represented by mol percentage, from 64.3 to67.3% of SiO₂, from 7.2 to 8.0% of Al₂O₃, from 4.5 to 8.5% of MgO, from0 to 8% of CaO, from 0.5 to 6.0% of SrO, from 0 to 2.0% of BaO, from 0to 2.0% of TiO₂, from 1.0 to 3.0% of ZrO₂, from 3.0 to 6.5% of Na₂O,from 2.5 to 6.3% of K₂O, and from 0.5 to 1.3% of La₂O₃.

Here, the measurement of small-angle X-ray scattering of the glass wascarried out as follows for the purpose of evaluating fine phaseseparation in the glass. Namely, using NANO-Viewer manufactured byRigaku K.K. as a small-angle X-ray scattering apparatus, and a imagingplate as a detector, X-ray was passed through a glass sample having athickness of about 0.1 mm and a size of about 1 cm×1 cm, and theobtained two-dimensional data was estimated by the averagecharacteristics of radiation emitted by a circular ring and thendata-conversion to obtain one-dimensional data, which was then subjectedto transmittance correction, in-air scattering correction andsample-thickness correction to obtain a scattered peak intensity. Thisscattered peak intensity has a correlation with the surface roughnessAFM-Ra of the glass at the time of the final polishing as describedhereinafter, as shown in FIG. 3.

In a case where a surface recording density of at least 1 Tb/inch² isdesired to be realized, such AFM-Ra is required to be adjusted to alevel of at most 0.15 nm, more preferably at most 0.12 nm. For such apurpose, for example from FIG. 3, it is evident that when AFM-Ra isdesired to be adjusted to a level of at most 0.15 nm, the abovescattered peak intensity is preferably adjusted to a level of at most220 (optional unit) or when AFM-Ra is desired to be adjusted to a levelof at most 0.12 nm, the above scattered peak intensity is preferablyadjusted to a level of at most 50 (optional unit). Accordingly, in sucha case, it is evident from FIG. 2 that (Mg+Ca)(La+Ti)/Zr is preferablyadjusted to be level of at most 12.0%. If (Mg+Ca)(La+Ti)/Zr exceeds12.0%, the compositional fluctuation due to phase separation tends to belarge, and the surface roughness at the time of the final polishing islikely to deteriorate, and it is more preferably at most 10.0%,particularly preferably at most 8.0%.

The glass of the present invention consists essentially of theabove-mentioned components, but may contain other components within arange not to impair the purpose of the present invention. In a casewhere such other components are contained, the total content of suchother components is preferably at most 5%, more preferably at most 2%.Now, such other components will be exemplified.

A clarifier such as SO₃, Cl, As₂O₃ or Sb₂O₃ may be contained in a totalamount of up to 1%.

Further, in a case where heating by irradiation with infrared ray iscarried out for heating a substrate, in order to increase the emissivityin the irradiation wavelength zone, a colorant such as Fe₂O₃, NiO, CoO,Cr₂O₃, CuO, MnO₂, CeO₂, Er₂O₃ or Yb₂O₃ may be contained in a totalamount of up to 0.5%.

ZnO may have an effect to lower the viscosity of the molten glassthereby to facilitate melting of the glass, or to lower T_(L), and insuch a case, it may be contained in an amount of up to 5%. If ZnOexceeds 5%, Tg is likely to be low, or d is likely to be large, and itis preferably at most 3%.

B₂O₃ may sometimes be contained in an amount of up to 5% in order toe.g. lower T_(L), but it may decrease the uniformity or phase separationcharacteristics of the glass, and in such a case, it is preferably notcontained.

EXAMPLES

In each of Examples 1 to 21, 28 to 50 and 52 to 65 in Tables 1 to 8,materials were prepared and mixed to obtain a composition shown by molpercentage in sections for from SiO₂ to La₂O₃ and by means of a platinumcrucible, melted at a temperature of from 1,550 to 1,650° C. for from 3to 5 hours. Typically, the mixture was melted at a temperature of from1,550 to 1,650° C. for from 1 to 2 hours, then stirred for 1 hour andfurther clarified for 1 hour. Then, the molten glass was cast to form aplate, followed by annealing.

With respect to a glass plate thus obtained, Tg (unit: ° C.), a (unit:10⁻⁷/° C.), the density d (unit: g/cm³) the Young's modulus E (unit:GPa), the specific modulus E/d (unit: MNm/kg), the temperature T₄ atwhich the viscosity becomes 10⁴ P=10⁴ dPa·s (unit: ° C.), the liquidphase temperature T_(L) (unit: ° C.), the phase separation property, andthe above-mentioned scattered peak intensity (optional unit) weremeasured or evaluated by the following methods. The results are shown inTables. In the Tables, “-” means that no measurement was carried out,and “*” means that the value was one obtained by calculation from thecomposition.

Tg: By means of a differential thermal dilatometer and using quartzglass as a reference sample, the rate of elongation of glass when it washeated from room temperature at a rate of 5° C./min to a temperature atwhich the glass softened and elongation no longer observed i.e. a yieldpoint, whereby a temperature corresponding to the critical point in thethermal expansion curve was taken as the glass transition temperature.

α: From the thermal expansion curve obtained in the same manner as theabove measurement of Tg, an average linear expansion coefficient withina range of from 50 to 350° C. was calculated.

d: Measured by an Archimedes method.

E: With respect to a glass plate having a thickness of from 4 to 10 mmand a size of about 4 cm×4 cm, the measurement was made by an ultrasonicpulse method.

T₄: Measured by means of a rotation viscometer.

T_(L): A glass specimen of about 1 cm×1 cm×0.8 cm was placed on aplatinum plate and heat-treated for 3 hours in an electric furnace setat every 10° C. within a temperature range of from 1,100 to 1,300° C.The glass was left to cool in atmospheric air and then observed by amicroscope, whereby the temperature range where crystals wereprecipitated was taken as a liquid phase temperature.

Phase separation property: A glass specimen of about 1 cm×1 cm×0.8 cmwas placed on a platinum plate and put in an electric furnace set at850° C. and 750° C. and maintained for about 20 minutes, whereupon itwas annealed at a cooling rate of −1° C./min or −10° C./min to roomtemperature, followed by irradiation with a high luminance power source,whereby whether or not turbidity was observed, was visually confirmed.As a result, one where no turbidity was observed in all tests wasidentified by ⊚, one where no turbidity was observed under theconditions of 750° C. and −1° C./min was identified by ◯, one where noturbidity was observed under the conditions of 750° C. and −10° C./minwas identified by Δ, and one where turbidity was observed in all testswas identified by X. The results are shown in the section for “Phaseseparation property 1” in the Tables.

Scattered peak intensity: As mentioned above, the measurement ofsmall-angle X-ray scattering of glass was carried out, and datatreatments were carried out to obtain the scattered peak intensity. Theresults are shown in the section for “Phase separation property 2” inthe Tables. As mentioned above, in a case where the after-mentionedAFM-Ra is desired to be adjusted to be at most 0.15 nm, this scatteredpeak intensity is preferably at most 220.

In Examples 22 to 27 and 51, no glass was prepared, and Tg, etc. wereobtained by calculation from the compositions.

Examples 1 to 52 represent Examples of the present invention, and amongthem, Examples 1 to 27 and 31 to 36 are examples for the glass 1, andExamples 9, 11, 16 and 28 to 48 are examples for the glass 2. Example 53is a Reference Example, and Examples 54 to 65 are Comparative Examples.

Tables 9 to 16 show the compositions of the glasses in Examples 1 to 65as represented by mass percentage.

The following test was carried out to measure the surface roughnessafter the polishing of each glass in Examples 6, 16, 28 to 31, 34, 37 to39, 41, 43, 44, 52 and 63 to 65.

As a test specimen, one obtained by finishing a glass plate having athickness of about 1.5 mm and a size of about 4 cm×4 cm by means of acerium oxide slurry so that the surface roughness would be from 0.3 to0.4 nm as measured by an atomic force microscope, was prepared.

Firstly, a suede abrasive cloth was mounted on a polishing plate of asmall single-side polishing machine manufactured by Hamai Co., Ltd. andsubjected to truing by means of a diamond dresser. This abrasive clothwas washed with pure water and by means of a brush, and then polishingoperation was carried out by supplying a colloidal silica slurry with anaverage particle size of 30 nm having the pH adjusted to 2. Thepolishing pressure was 10 kPa, the sun gear rotational speed was 40 rpm,and the polishing time was 20 minutes.

After the polishing, each glass was washed with pure water and analkaline cleanser and then dried by blowing dry air thereto.

The surface roughness Ra of each glass thus obtained was measured bymeans of an atomic force microscope SPA400, manufactured by SIINanoTechnology Inc. The results are shown in the section for “Surfaceroughness” (unit: nm) in the Tables. The surface roughness Ra thusobtained is referred to as AFM-Ra in this specification.

FIG. 3 is one wherein the above scattered peak intensity (phaseseparation property 2) and this AFM-Ra were plotted. From this Fig., itis evident that a positive correlation exists between the two.

TABLE 1 Examples 1 2 3 4 5 6 7 8 SiO₂ 57.5 60.0 60.0 62.5 63.0 63.5 65.065.0 Al₂O₃ 7.5 7.5 6.5 7.5 5.5 5.5 5.0 5.0 MgO 2.0 2.0 6.0 2.0 6.5 3.62.0 2.0 CaO 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 SrO 7.0 7.0 7.0 7.0 3.2 6.87.0 7.0 BaO 10.0 7.5 5.5 5.0 5.5 5.0 5.0 5.0 ZnO 0 0 0 0 0 0 0 0 TiO₂1.5 1.5 2.0 1.5 1.5 1.5 1.5 1.5 ZrO₂ 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0Li₂O 0 0 0 0 0 0 0 0 Na₂O 4.0 4.0 1.0 4.0 1.3 1.1 1.5 4.0 K₂O 4.0 4.05.0 4.0 7.0 6.5 6.5 4.0 La₂O₃ 1.5 1.5 2.0 1.5 1.5 1.5 1.5 1.5 RO 21.018.5 20.5 16.0 17.2 17.4 16.0 16.0 R₂O 8.0 8.0 6.0 8.0 8.3 7.6 8.0 8.0LiNa/K 1.00 1.00 0.20 1.00 0.19 0.17 0.23 1.00 Al(ZrKLa)/RO 3.04 3.453.17 3.98 3.68 3.48 3.44 2.66 LaTi•CaMg/Zr 4 4 10.67 4 8.5 5.6 4 4 Tg682 691 736 697 711 716 707 681 α 84 82 75 79 80 78 79 80 d 3.22 3.123.10 3.01 2.95 3.00 3.00 3.00 E 83.4* 82.8* 85.9* 82.1* 81.8 81.6 83.181.2* E/d 25.9* 26.6* 27.7* 27.3* 27.7 27.2 27.7 27.0* T₄ 1117 1132 11501164 1173 1174 1166 1142 T_(L) 1110 1130 1150 1160 1170 1170 1120 <1100Phase — — — — — ⊚ — — separation property 1 Phase — — — — — 43 — —separation property 2 Surface — — — — — 0.13 — — roughness

TABLE 2 Examples 9 10 11 12 13 14 15 16 SiO₂ 66.5 67.5 64.0 63.5  62.5 63.05 63.3 63.5 Al₂O₃ 4.0 5.0 7.0 6.5 7.0 7.0 6.8 7.0 MgO 5.0 2.0 7.07.5 8.5 8.1 8.3 8.0 CaO 0 2.0 6.0 4.0 4.0 4.7 5.5 5.0 SrO 7.0 7.0 4.05.5 5.0 3.7 2.7 3.3 BaO 4.0 2.5 1.0 1.0 1.0 1.0 1.0 1.0 ZnO 0 0 0 0  0   0 0 0 TiO₂ 1.0 1.5 1.0 1.5 1.5 1.5 1.5 1.0 ZrO₂ 2.5 3.0 1.25 1.5 1.51.5 1.5 1.65 Li₂O 0 0 0 0   0   0 0 0 Na₂O 3.0 4.0 2.0 3.0 2.5 2.6 2.72.6 K₂O 5.0 4.0 5.75 5.0 5.5 5.6 5.5 5.7 La₂O₃ 2.0 1.5 1.0 1.0 1.0 1.251.2 1.25 RO 16.0 13.5 18.0 18.0  18.5  17.5 17.5 17.3 R₂O 8.0 8.0 7.758.0 8.0 8.2 8.2 8.3 LiNa/K 0.60 1.00 0.35  0.60  0.45 0.46 0.49 0.46Al(ZrKLa)/RO 2.38 3.15 3.11  2.71  3.03 3.34 3.19 3.48 LaTi•CaMg/Zr 6 420.8 19.17 20.83 23.47 24.84 17.73 Tg 687 690 708 695    709    710 710711 α 76 77 78 79   75   78 79 78 d 2.98 2.90 2.74  2.78  2.78 2.77 2.752.76 E 83.4* 80.6* 81.2 82.0  82.1  81.6 81.8 81.5 E/d 25.9* 27.8* 29.629.4  29.5  29.5 29.8 29.6 T₄ 1146 1176 1184 1154*    1167*    1170 11471177 T_(L) 1130 1140 1160 1120    1170    1140 1145 1160 Phase — — — — —◯ Δ ◯ separation property 1 Phase — — — — — — — 843 separation property2 Surface — — — — — — — 0.17 roughness

TABLE 3 Examples 17 18 19 20 21 22 23 24 SiO₂ 63.7 62.5 65.0  63.0 63.064.0  64.5  63.5  Al₂O₃ 6.5 6.5 7.5 6.5 6.5 7.0 5.5 5.5 MgO 4.0 7.0 2.03.5 6.0 7.0 0   3.0 CaO 2.0 1.0 2.0 2.0 0 5.0 3.0 2.0 SrO 7.0 2.0 7.05.0 6.5 6.0 7.8 7.6 BaO 3.0 7.0 2.5 4.5 3.5 1.5 5.0 5.9 ZnO 0 0 0   0 00   0   0   TiO₂ 1.5 1.0 1.5 2.0 2.0 0   1.5 1.5 ZrO₂ 2.6 2.5 3.0 3.52.5 1.0 2.5 3.0 Li₂O 0 0 0   0 0 0   0   0   Na₂O 1.5 2.0 4.0 2.0 1.51.8 2.2 0   K₂O 6.7 6.5 4.0 6.0 6.5 5.7 6.5 6.5 La₂O₃ 1.5 2.0 1.5 2.02.0 1.0 1.5 1.5 RO 16.0 17.0 13.5  15.0 16.0 19.5  15.8  18.5  R₂O 8.28.5 8.0 8.0 8.0 7.5 8.7 6.5 LiNa/K 0.22 0.31  1.00 0.33 0.23  0.32  0.340   Al(ZrKLa)/RO 4.39 4.21  4.72 4.98 4.47  2.76  3.66  3.27LaTi•CaMg/Zr 6.92 9.6 4   6.29 9.6 12   3.6 5   Tg 702 711 701*   727725 705*   694*   730*   α 79.5* 81.0 74.3* 77.9 78.1 77.7* 82.4* 75.7*d 2.92* 2.99*  2.90* 3.00* 2.96*  2.78*  3.00*  3.05* E 80.4* 80.7*81.5* 82.2* 81.8* 80.3* 78.1* 81.3* E/d 27.5* 26.9* 28.1* 27.4* 27.6*28.8* 26.0* 26.6* T₄ 1172 1178 1182*    1171 1156 1168*    1156*   1174*    T_(L) 1210 1220 1230    1200 1220 Phase — — — — — separationproperty 1 Phase — — — — — separation property 2 Surface — — — — —roughness

TABLE 4 Examples 25 26 27 28 29 30 31 32 SiO₂ 64.55 67.0  69.0  66.567.0 65.0 65.0 65.4 Al₂O₃ 6.0 5.0 4.0 7.5 7.5 7.5 7.2 7.4 MgO 3.2 2.54.5 6.0 6.0 6.0 8.0 7.8 CaO 2.0 2.0 2.0 6.0 6.0 6.0 4.0 4.7 SrO 4.5 6.12.5 2.5 2.5 2.5 2.5 1.8 BaO 5.0 3.0 1.0 0 0 0 2.0 0.8 ZnO 0   0   0   00 0 0 0 TiO₂ 1.5 1.5 2.0 0.5 0 0 0 0 ZrO₂ 3.0 3.0 3.0 1.0 1.0 3.0 1.71.8 Li₂O 2.5 0   0   0 0 0 0 0 Na₂O 1.0 4.9 4.5 4.0 4.0 4.0 3.0 4.2 K₂O5.0 3.5 5.5 5.0 5.0 5.0 5.6 5.0 La₂O₃  1.75 1.5 2.0 1.0 1.0 1.0 1.0 1.1RO 14.7  13.6  10.0  14.5 14.5 14.5 16.5 15.1 R₂O 8.5 8.4 10.0  9.0 9.09.0 8.6 9.2 LiNa/K  0.70  1.40  0.82 0.80 0.80 0.80 0.54 0.84Al(ZrKLa)/RO  3.98  2.94 4.2 3.62 3.62 4.66 3.62 3.87 LaTi•CaMg/Zr  5.634.5 8.7 18 12 4 7.06 7.64 Tg 680*   683*   688*   703 703 722 696 694 α75.1* 75.3* 77.7* 77 77 76 76 76 d  2.97*  2.90*  2.79* 2.65 2.64 2.702.74 2.69 E 83.4* 80.9* 79.0* 80.4 79.8 83.1 81.2 81.9 E/d 28.1* 27.9*28.4* 30.4 30.3 30.8 29.7 30.4 T₄ 1144*    1156*    1171*    — — — 1195— T_(L) 1160 1160 >1200 <1100 1160 Phase — — — — — separation property 1Phase 245 210 86 65 — separation property 2 Surface 0.16 0.14 0.14 0.13— roughness

TABLE 5 Examples 33 34 35 36 37 38 39 40 SiO₂ 64.3 66.0 66.5 66.5 66.566.5 65.8 65.9 Al₂O₃ 7.4 7.4 7.4 7.4 7.7 7.4 8.0 7.5 MgO 8.0 5.0 6.5 6.57.0 7.0 5.5 6.5 CaO 5.8 5.0 2.5 0 1.0 1.0 0 0 SrO 1.7 3.0 3.5 6.0 4.04.0 6.0 5.9 BaO 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 ZnO 0 0 0 0 0 0 0 0 TiO₂0 0 0 0 0 0 0 0 ZrO₂ 1.8 2.5 2.0 2.0 2.0 2.0 2.5 2.5 Li₂O 0 0 0 0 0 0 00 Na₂O 4.2 4.5 4.5 4.5 4.3 4.3 4.3 4.0 K₂O 5.45 5.5 5.5 5.5 5.9 5.9 6.36.3 La₂O₃ 0.75 0.5 1.0 1.0 1.0 1.3 1.0 0.8 RO 16.1 13.6 13.1 13.1 12.612.6 12.1 13.0 R₂O 9.65 10.0 10.0 10.0 10.2 10.2 10.6 10.3 LiNa/K 0.770.82 0.82 0.82 0.73 0.73 0.68 0.63 Al(ZrKLa)/RO 3.68 4.63 4.80 4.80 5.445.40 6.48 5.54 LaTi•CaMg/Zr 5.75 2 4.5 3.25 4 5.2 2.2 2.08 Tg 689 684685 688 691 693 696 694 α 80 78 78 78 79 79 80 77 d 2.66 2.67 2.70 2.732.69 2.71 2.74 2.72 E 81.2 80.6 80.4 79.8 79.5 79.9 78.9 79.5 E/d 30.530.2 29.8 29.2 29.6 29.5 28.8 29.2 T₄ — — — — — — — — T_(L) 1170 11401120 <1100 1120 1160 1180 1140 Phase — — — — — — — — separation property1 Phase — 21 114 29 72 — 33 17 separation property 2 Surface — 0.10 — —0.12 0.11 0.10 — roughness

TABLE 6 Examples 41 42 43 44 45 46 47 48 SiO₂ 66.3 66.1 66.0 65.8 66.067.0 67.3 66.4 Al₂O₃ 7.7 7.6 7.2 7.5 7.5 7.5 7.4 7.5 MgO 4.5 4.7 8.0 8.08.5 5.0 5.5 5.6 CaO 4.5 4.7 6.0 6.0 6.0 3.5 5.5 5.6 SrO 3.2 3.2 1.0 1.00.5 4.2 2.0 2.3 BaO 0 0.2 0 0 0 0 0 0 ZnO 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 00 0 0 ZrO₂ 2.5 2.5 1.8 1.7 1.5 2.5 2.0 2.2 Li₂O 0 0 0 0 0 0 0 0 Na₂O 4.64.7 6.5 6.5 6.5 6.0 5.8 5.7 K₂O 6.0 5.6 2.5 2.5 2.5 3.5 3.7 3.8 La₂O₃0.7 0.7 1.0 1.0 1.0 0.8 0.8 0.9 RO 12.2 12.8 15.0 15.0 15.0 12.7 13.013.5 R₂O 10.6 10.3 9.0 9.0 9.0 9.5 9.5 9.5 LiNa/K 0.77 0.84 2.6 2.6 2.61.71 1.57 1.50 Al(ZrKLa)/RO 5.81 5.23 2.54 2.60 2.50 4.02 3.70 3.83LaTi•CaMg/Zr 2.52 2.63 7.78 8.24 9.67 2.72 4.4 4.58 Tg 690 695 689 694694 694 694 690 α 80 81 75 75 75 76 77 78 d 2.66 2.67 2.65 2.65 2.632.68 2.63 2.66 E 80.2 80.4 83.4 83.0 83.2 80.5 80.8 82.1 E/d 30.2 30.231.5 31.4 31.6 30.0 30.7 30.9 T₄ 1236 — — 1165 — — — — T_(L) 1150 11501170 1170 1190 1160 1150 1140 Phase — — — — — — — — separation property1 Phase 31 — 92 — — 30 57 — separation property 2 Surface 0.10 — 0.130.12 — — — — roughness

TABLE 7 Examples 49 50 51 52 53 54 55 56 SiO₂ 63.0  63.0  67.5  65.062.5  60.0  57.5  62.5  Al₂O₃ 7.5 7.5 5.0 7.5 10.0  10.0  10.0  6.0 MgO6.0 7.5 2.0 6.0 2.0 2.0 2.0 4.0 CaO 5.0 5.0 2.0 6.0 2.0 2.0 2.0 1.5 SrO 1.75  1.75 7.0 2.5 7.0 7.0 7.0 5.5 BaO 1.5 1.5 2.5 0 2.5 5.0 7.5 5.0ZnO 0   0   0   0 0   0   0   0   TiO₂ 1.5 1.5 1.5 2.0 1.5 1.5 1.5 2.0ZrO₂ 3.0 1.5 3.0 1.0 3.0 3.0 3.0 4.0 Li₂O 1.5 1.5 0   0 0   0   0   0  Na₂O 2.0 2.0 4.8 4.0 4.0 4.0 4.0 2.0 K₂O  6.25  6.25 3.2 5.0 4.0 4.0 4.05.5 La₂O₃ 1.0 1.0 1.5 1.0 1.5 1.5 1.5 2.0 RO 14.25 15.75 13.5  14.513.5  16.0  18.5  16.0  R₂O  9.75  9.25 8.0 9.0 8.0 8.0 8.0 7.5 LiNa/K 0.56  0.56  1.50 0.80  1.00  1.00  1.00  0.36 Al(ZrKLa)/RO  5.92  4.17 2.85 3.62  6.30  5.31  4.59  4.31 LaTi•CaMg/Zr  9.17 20.83 4   36 4  4   4   5.5 Tg 666    663    679*   702 715*   714*   713*   724*   α75.2  81.5  77.0* 83 74.0* 77.4* 80.7* 76.8* d  2.67  2.73  2.90* 2.67 2.91*  3.02*  3.12*  3.05* E 81.9  82.1  81.2* 81.0 82.4* 83.0* 83.7*83.8* E/d 30.7  30.1  28.0* 30.4 28.3* 27.5* 26.8* 27.5* T₄ 1210*   1166*    1158*    — 1197*    1170*    1150*    1159*    T_(L) >1300    1190    1160 1240    1280    1230    1270    Phase — — — — — — —separation property 1 Phase — — 671 — — — — separation property 2Surface — — 0.20 — — — — roughness

TABLE 8 Examples 57 58 59 60 61 62 63 64 65 SiO₂ 61.25 55.0  55.0  58.5 56.5 57.5  66.5 60.0 65.0 Al₂O₃ 7.5 14.0  10.0  12.0  12.0 11.0  4.7 8.57.5 MgO 7.0 2.0 3.0 3.0 2.0 2.0 3.4 7.0 6.0 CaO 8.0 2.0 5.0 3.0 2.0 2.06.2 4.5 8.0 SrO  2.25  4.75 8.0 7.0 6.5 5.0 4.7 7.0 2.5 BaO 0   8.0 0  4.0 8.0 8.0 3.6 0 0 ZnO 0   0   0   0   0 2.0 0 0 0 TiO₂ 1.5 2.0 3.5 0  2.0 1.0 0 3.5 0 ZrO₂ 3.0 2.5 2.0 3.0 2.5 2.0 1.7 1.0 1.0 Li₂O 0.8 0  0   0   0 0   0 0 0 Na₂O 1.2 3.0 3.5 1.0 2.75 3.0 4.8 4.0 4.0 K₂O 6.55.0 5.0 6.0 4.0 4.0 4.4 4.5 5.0 La₂O₃ 1.0  1.75 5.0 2.5 1.75 2.5 0 0 1.0RO 17.25 16.75 16.0  17.0  18.5 17.0  17.9 18.5 16.5 R₂O 8.5 8.0 8.5 7.06.75 7.0 9.2 8.5 9.0 LiNa/K  0.31 0.6 0.7  0.17 0.69  0.75 1.09 0.890.80 Al(ZrKLa)/RO  4.57  7.73 7.5  8.12 5.35 5.5 1.6 2.53 3.18LaTi•CaMg/Zr 12.5  6   34   5   6 7   0 40.25 14 Tg 709    741    739   768    733 717    691 691 698 α 76.9  79.0  84.8  75.4  78.7 78.6  81 8180 d  2.74  3.08  2.96  3.05 3.13  3.18 2.77 2.71 2.67 E 85.4  83.7 86.4  82.9  86.3* 82.8  76.5 82.4 81.3 E/d 31.2  27.1  29.2  27.2  27.2*26.1  27.6 30.4 30.5 T₄ 1178*    1203*    1019*    1169*    11731175*    1145 1147 — T_(L) >1300     >1300     >1300     >1300     >13001290    1080 1140 1160 Phase X — — — — — — — — separation property 1Phase — — — — — — 12 954 352 separation property 2 Surface — — — — — —0.10 0.17 0.18 roughness

TABLE 9 Examples 1 2 3 4 5 6 7 8 SiO₂ 41.75 44.83 45.61 48.10 49.7249.14 50.17 50.70 Al₂O₃ 9.24 9.51 8.38 9.79 7.37 7.22 6.55 6.62 MgO 0.971.00 3.06 1.03 3.44 1.87 1.04 1.05 CaO 1.36 1.39 1.42 1.44 1.47 1.441.44 1.46 SrO 8.77 9.02 9.18 9.29 4.36 9.08 9.32 9.42 BaO 18.53 14.3010.67 9.82 11.08 9.88 9.85 9.95 ZnO 0 0 0 0 0 0 0 0 TiO₂ 1.45 1.49 2.021.54 1.57 1.54 1.54 1.56 ZrO₂ 4.47 4.60 4.68 4.73 4.86 4.76 4.75 4.80Li₂O 0 0 0 0 0 0 0 0 Na₂O 3.00 3.08 0.78 3.18 1.06 0.88 1.19 3.22 K₂O4.55 4.69 5.96 4.83 8.66 7.89 7.87 4.89 La₂O₃ 5.91 6.08 8.24 6.26 6.426.30 6.28 6.34

TABLE 10 Examples 9 10 11 12 13 14 15 16 SiO₂ 52.30 54.29 55.16 54.3853.56 53.89 54.69 54.38 Al₂O₃ 5.34 6.83 10.24 9.45 10.18 10.15 9.9710.17 MgO 2.64 1.08 4.05 4.31 4.89 4.64 4.81 4.60 CaO 0 1.50 4.83 3.203.20 3.75 4.44 4.00 SrO 9.49 9.71 5.95 8.12 7.39 5.45 4.02 4.87 BaO 8.035.13 2.20 2.19 2.19 2.18 2.21 2.19 ZnO 0 0 0 0 0 0 0 0 TiO₂ 1.05 1.601.15 1.71 1.71 1.71 1.72 1.14 ZrO₂ 4.03 4.95 2.21 2.63 2.64 2.63 2.662.90 Li₂O 0 0 0 0 0 0 0 0 Na₂O 2.43 3.32 1.78 2.65 2.21 2.29 2.41 2.30K₂O 6.17 5.04 7.77 6.71 7.39 7.50 7.45 7.65 La₂O₃ 8.53 6.54 4.67 4.644.65 5.79 5.62 5.81

TABLE 11 Examples 17 18 19 20 21 22 23 24 SiO₂ 45.45 42.30 50.53 47.8944.72 46.73 44.04 43.78 Al₂O₃ 16.10 18.27 8.75 8.45 13.20 14.42 13.0015.50 MgO 1.59 1.03 2.13 3.60 1.57 1.71 1.54 1.53 CaO 2.21 1.44 1.480.72 3.63 3.97 3.57 3.55 SrO 10.91 6.30 9.58 2.64 10.73 11.72 10.5710.50 BaO 0 15.70 6.07 13.69 0 0 0 0 ZnO 0 0 0 0 0 0 0 0 TiO₂ 1.58 2.051.58 1.02 3.62 3.96 1.53 1.52 ZrO₂ 6.48 3.94 4.23 3.93 3.19 3.49 3.143.12 Li₂O 0 0 0 0 0 0 0 0 Na₂O 2.85 2.38 1.23 1.58 2.81 3.07 2.77 2.75K₂O 6.20 6.03 8.33 7.81 6.10 6.66 6.00 5.97 La₂O₃ 10.72 7.30 6.45 8.3110.54 23.04 18.69 10.32

TABLE 12 Examples 25 26 27 28 29 30 31 32 SiO₂ 45.27 46.36 47.73 58.7159.24 56.42 55.78 57.09 Al₂O₃ 13.36 16.42 16.91 11.24 11.25 11.05 10.4910.96 MgO 1.58 2.70 2.78 3.55 3.56 3.49 4.61 4.57 CaO 3.67 0.75 0 4.944.95 4.86 3.20 3.83 SrO 10.86 11.12 11.45 3.81 3.81 3.74 3.70 2.71 BaO 00 0 0 0 0 4.38 1.78 ZnO 0 0 0 0 0 0 0 0 TiO₂ 1.57 1.61 1.66 0.59 0 0 0 0ZrO₂ 6.46 6.61 6.81 1.81 1.81 5.34 2.99 3.22 Li₂O 0 0 0 0 0 0 0 0 Na₂O2.84 2.91 3.00 3.64 3.65 3.58 2.66 3.78 K₂O 6.17 6.32 6.51 6.92 6.936.80 7.54 6.84 La₂O₃ 10.67 10.93 15.76 4.79 4.79 4.71 4.65 5.21

TABLE 13 Examples 33 34 35 36 37 38 39 40 SiO₂ 57.03 57.70 57.27 56.3156.92 56.38 54.92 55.77 Al₂O₃ 11.14 10.98 10.82 10.63 11.18 10.65 11.3310.77 MgO 4.76 2.93 3.76 3.69 4.02 3.98 3.08 3.69 CaO 4.80 4.08 2.01 00.80 0.79 0 0 SrO 2.60 4.52 5.20 8.76 5.90 5.85 8.64 8.61 BaO 1.36 1.341.32 1.30 1.31 1.30 1.28 1.30 ZnO 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0ZrO₂ 3.27 4.48 3.53 3.47 3.51 3.48 4.28 4.34 Li₂O 0 0 0 0 0 0 0 0 Na₂O3.84 4.06 4.00 3.93 3.80 3.76 3.70 3.49 K₂O 7.58 7.54 7.43 7.30 7.927.84 8.24 8.36 La₂O₃ 3.61 2.37 4.67 4.59 4.64 5.98 4.53 3.67

TABLE 14 Examples 41 42 43 44 45 46 47 48 SiO₂ 57.56 57.42 59.65 59.4159.99 58.40 59.90 58.60 Al₂O₃ 11.35 11.20 11.04 11.49 11.57 11.09 11.1811.23 MgO 2.62 2.74 4.85 4.85 5.18 2.92 3.28 3.32 CaO 3.65 3.81 5.065.06 5.09 2.85 4.57 4.61 SrO 4.79 4.79 1.56 1.56 0.78 6.31 3.07 3.50 BaO0 0.44 0 0 0 0 0 0 ZnO 0 0 0 0 0 0 0 0 TiO₂ 0 0 0 0 0 0 0 0 ZrO₂ 4.454.45 3.34 3.15 2.80 4.47 3.65 3.98 Li₂O 0 0 0 0 0 0 0 0 Na₂O 4.12 4.216.06 6.05 6.10 5.39 5.33 5.19 K₂O 8.17 7.63 3.54 3.54 3.56 4.78 5.165.26 La₂O₃ 3.30 3.30 4.90 4.90 4.93 3.78 3.86 4.31

TABLE 15 Examples 49 50 51 52 53 54 55 56 SiO₂ 49.81 50.70 46.64 57.1446.55 45.18 43.89 43.76 Al₂O₃ 17.34 17.65 15.15 11.19 15.93 15.46 15.0215.50 MgO 1.71 1.74 1.09 3.54 1.57 1.53 1.48 1.53 CaO 2.38 2.43 1.514.92 2.19 2.13 2.07 2.13 SrO 10.28 10.46 9.80 3.79 8.10 5.24 5.09 5.25BaO 4.35 8.85 16.56 0 0 0 3.77 3.88 ZnO 0 0 0 0 0 0 0 0 TiO₂ 0 0 1.072.34 1.56 1.51 1.47 1.52 ZrO₂ 5.24 5.33 3.33 1.80 6.42 6.23 6.05 6.24Li₂O 0 0 0 0 0 0 0 0 Na₂O 2.64 0.89 2.51 3.63 2.82 2.74 2.66 4.32 K₂O8.01 8.15 5.09 6.89 6.13 8.33 5.78 5.97 La₂O₃ 11.54 11.75 8.75 4.7710.61 10.29 10.00 10.32

TABLE 16 Examples 57 58 59 60 61 62 63 64 65 SiO₂ 49.82 41.85 41.1443.21 42.06 42.07 57.74 52.97 57.54 Al₂O₃ 16.17 13.58 13.36 14.03 13.6613.66 6.93 12.73 11.27 MgO 1.16 0.98 0.96 1.01 0.98 0.98 1.98 4.14 3.56CaO 1.62 1.36 1.34 1.40 1.37 1.37 5.02 3.71 6.61 SrO 10.46 8.79 6.179.07 8.83 8.83 7.04 10.66 3.82 BaO 17.69 14.86 14.61 11.51 11.20 11.207.98 0 0 ZnO 0 0 1.94 2.04 1.98 1.98 0 0 0 TiO₂ 1.15 0.97 0.95 1.00 0.970.97 0 4.11 0 ZrO₂ 3.55 2.99 2.93 3.08 1.50 0 3.03 1.81 1.82 Li₂O 0 0 00 0 0 0 0 0 Na₂O 2.68 2.25 2.21 2.33 2.26 2.26 4.30 3.64 3.65 K₂O 5.434.56 4.49 4.71 4.59 4.59 5.99 6.23 6.94 La₂O₃ 11.75 5.94 9.70 10.19 9.929.92 0 0 4.80

INDUSTRIAL APPLICABILITY

The present invention is useful for a data recording medium, itssubstrate and their production.

The entire disclosure of Japanese Patent Application No. 2008-16550filed on Jan. 28, 2008 including specification, drawings and summary isincorporated herein by reference in its entirety.

1. Glass for a data storage medium substrate, which comprises, asrepresented by mol percentage based on the following oxides, from 55 to70% of SiO₂, from 2.5 to 9% of Al₂O₃, from 0 to 10% of MgO, from 0 to 7%of CaO, from 1 to 10% of SrO, from 0.5 to 12.5% of BaO, from 0 to 2.5%of TiO₂ from 0.5 to 3.7% of ZrO₂, from 0 to 2.5% of Li₂O, from 0 to 5%of Na₂O, from 2 to 8% of K₂O and from 0.5 to 5% of La_(2 O) ₃, providedthat the total content of Al₂O₃ and ZrO₂ (Al₂O₃+ZrO₂) is at most 12%,and the total content of Li₂O, Na₂O and K₂O (R₂O) is at most 10%,wherein a difference obtained by subtracting the Li₂O content from thetotal content of SrO and BaO is at least 2 mol %.
 2. The glass for adata storage medium substrate according to claim 1, wherein the molratio of the total content of Li₂O and Na₂O to the K₂O content((Li₂O+Na₂O)/K₂O) is at most 1.4.
 3. The glass for a data storage mediumsubstrate according to claim 1, wherein the total content of MgO, CaO,SrO and BaO is at least 10 mol %.
 4. The glass for a data storage mediumsubstrate according to claim 1, wherein a value obtained by dividing aproduct of the Al₂O₃ content multiplied by the total content of ZrO₂,K₂O and La₂O₃, by the total content of MgO, CaO, SrO and BaO(Al₂O₃×(ZrO₂+K₂O+La₂O₃)÷(MgO+CaO+SrO+BaO)) is at most 4.0 mol %.
 5. Theglass for a data storage medium substrate according to claim 1, whereinthe total content of Al₂O₃ and ZrO₂ is at most 9 mol %, and the totalcontent of Al₂O₃, ZrO₂, K₂O and La₂O₃ is at most the total content ofMgO, CaO, SrO and BaO.
 6. The glass for a data storage medium substrateaccording to claim 1, wherein the total content of Al₂O₃ and ZrO₂ ismore than 9 mol %, and the total content of ZrO₂ and K₂O is less than 9mol %.
 7. The glass for a data storage medium substrate according toclaim 1, wherein the total content of MgO, CaO and TiO₂ is at most 16mol %.
 8. The glass for a data storage medium substrate according toclaim 1, wherein the content of Al₂O₃ is, as represented by masspercentage, less than 11%.
 9. A glass substrate for a data storagemedium, which is made of the glass for a data storage medium substrateas defined in claim
 1. 10. A magnetic disk having a magnetic recordinglayer formed on the glass substrate for a data storage medium as definedin claim
 9. 11. The glass for a data storage medium substrate accordingto claim 1, which has a Young's modulus of 75 to 90 GPa.